Concepts: MEANS Procedure

is equivalent to

proc means; 
 class a b c d e; 
 types a*b a*c a*d a*e b*c b*d b*e c*d c*e d*e 
       a*b*c a*b*d a*b*e a*c*d a*c*e a*d*e 
       b*c*d b*c*e c*d*e; 
 run;

If you omit the TYPES statement and the WAYS statement, then PROC MEANS uses all class variables to subgroup the data (the NWAY type) for displayed output and computes all types (2 ^k ) for the output data set.

Ordering the Class Values

PROC MEANS determines the order of each class variable in any type by examining the order of that class variable in the corresponding one-way type. You see the effect of this behavior in the options ORDER=DATA or ORDER=FREQ. When PROC MEANS subdivides the input data set into subsets , the classification process does not apply the options ORDER=DATA or ORDER=FREQ independently for each subgroup. Instead, one frequency and data order is established for all output based on an nonsubdivided view of the entire data set. For example, consider the following statements:

data pets; 
 input Pet $ Gender $; 
 datalines; 
dog  m 
dog  f 
dog  f 
dog  f 
cat  m 
cat  m 
cat  f 
; 

proc means data=pets order=freq; 
   class pet gender; 
run;

The statements produce this output.

The SAS System                               1 

    The MEANS Procedure 

                           N 
Pet         Gender       Obs 
----------------------------
dog         f              3 

            m              1 

cat         f              1 

            m              2 
----------------------------

In the example, PROC MEANS does not list male cats before female cats. Instead, it determines the order of gender for all types over the entire data set. PROC MEANS found more observations for female pets (f=4, m=3).

Computational Resources

PROC MEANS employs the same memory allocation scheme across all operating environments. When class variables are involved, PROC MEANS must keep a copy of each unique value of each class variable in memory. You can estimate the memory requirements to group the class variable by calculating

where

When you use the GROUPINTERNAL option in the CLASS statement, Lc _i is simply the unformatted length of c _i .

Each unique combination of class variables, c _{1 i} c _{2 i} , for a given type forms a level in that type (see TYPES Statement on page 546). You can estimate the maximum potential space requirements for all levels of a given type, when all combinations actually exist in the data (a complete type), by calculating

where

Clearly, the memory requirements of the levels overwhelm those of the class variables. For this reason, PROC MEANS may open one or more utility files and write the levels of one or more types to disk. These types are either the primary types that PROC MEANS built during the input data scan or the derived types.

If PROC MEANS must write partially complete primary types to disk while it processes input data, then one or more merge passes may be required to combine type levels in memory with those on disk. In addition, if you use an order other than DATA for any class variable, then PROC MEANS groups the completed types on disk. For this reason, the peak disk space requirements can be more than twice the memory requirements for a given type.

When PROC MEANS uses a temporary work file, you will receive the following note in the SAS log:

Processing on disk occurred during summarization. 
Peak disk usage was approximately

nnn

Mbytes. 
Adjusting SUMSIZE may improve performance.